How could we know what proof the DA has?
With a subpoena to the ISP and a record of the IP address, they can probably trace it back to your friend. If they are going through all this trouble then there must be a little more to it than simply giving out a number ... I wonder if some offers were made in her name to the effect of, "For a good time call me at ...."
Normally I can't see a DA wasting his time with a chat room prank ... something more is going on here if they are actually going ahead with this.
- Carl